Stroke is the reportedly third leading cause of death in the United States and other developed countries. It is a major source of disability that often leads to hospitalization and long-term care. There are reported to be more than 100,000 new strokes each year in the UK (the incidence in the US is reportedly similar). Over 80% of strokes are as a result of cerebral infarct and of those, 23% die within one year and 65% gain functional independence. Approximately 75% of middle cerebral artery infarcts result in motor deficit, particularly of the upper limb and 24% of patients have residual upper limb motor loss at three months post-stroke. Various longitudinal studies have investigated the long-term outcome following stroke. For example, it is reported that for 30% to 60% of patients, the paretic arm remains without function and that half of all acute stroke patients starting rehabilitation will have a marked impairment of function of one arm of whom only about 14% will regain useful upper limb function.
Following a stroke, almost all patients undergo a period of rehabilitation (Physiotherapy, Occupational Therapy, Speech and Language Therapy) as appropriate. Although there is now overwhelming evidence that patients treated in specialist stroke units have a better outcome than those treated in general hospitals, there is no conclusive evidence that one conventional physical or occupational therapy is more effective than another, and there has been no conclusive evidence to support the effectiveness of conventional therapy in the treatment of upper limb impairment following stroke.
Unlike tetraplegic patients, whose motor impairment is specifically defined by the level and extent of the injury and whose main problem is grasping due to weak finger flexion, stroke patients, because of the nature of the lesion, have a more complex and varied pattern of motor impairment. Firstly, stroke patients are often unable to grip because of weak wrist extensors rather than an inability to activate finger flexors, and it is often spasticity of the flexor muscles that prevents opening of the hand, thus functionally impairing performance. To perform an effective power grip, or even to manipulate objects, requires the wrist to be held in a functional position of slight extension maintained by activity in the wrist extensors, mainly Extensor Carpi Radialis (ECR) and Extensor Carpi Ulnaris (ECU).
Surface Functional Electrical Stimulation (FES) systems have been used to strengthen wrist extensors and some studies have measured improvement both in muscle strength (grip and wrist extension) and hand function. When stimulation is activated voluntarily, usually by the electromyography (EMG) signal from either the target or a remote muscle, improvement in function has been greater. Achieving a functional movement with surface FES however is rarely possible due to the anatomical arrangement of the extensor muscles of the forearm. ECR and ECU lie deep to the finger extensors, so that in practice it is extremely difficult to activate the wrist extensors without also activating the finger extensors; thus preventing the patient from performing a functional grip. Motor re-learning research has demonstrated that achievement and repetition of functional tasks is essential to effect neuroplastic changes, therefore, although surface FES may improve strength, reduce spasticity and increase range of movement, because it is difficult to stimulate a functional movement, it may be less effective in motor-relearning.
Secondly, in addition to weakness of wrist extension, two other common problems seen following stroke are functionally important: (1) the inability to extend the elbow, and (2) the inability to extend the thumb, because of a combination of weakness and spasticity. Weakness of triceps brachii muscle and spasticity of biceps brachii affect elbow movement and weakness of Extensor Pollicis Longus (EPL) muscle and spasticity in the thumb flexors and opponens pollicis affect thumb movement. Like the wrist extensors, EPL is a difficult muscle to activate with surface stimulation as, for most of its length it lies deep to Extensor Pollicis Brevis (EPB). Inability to effectively extend the elbow considerably reduces the work space in which the individual can function, so that even if they have wrist and finger control, function is impaired. Inability to bring the thumb away from the hand restricts the ability to open the hand for grasping.
During the last decade there have been some substantial developments in Functional Electrical Therapy (FET) and Therapeutic Electrical Stimulation (TES) systems and they have been more widely used and evaluated. Multi-channel surface stimulation devices have been reported in the recent literature. Exemplary of such systems is the Ness Handmaster Neuroprosthesis which combines a plastic splint to provide wrist stabilization and housing for the electrodes that stimulate paralyzed muscles in the forearm and the hand muscle groups. The device was originally designed for patients who have no voluntary control of the wrist or hand, such as tetraplegic patients who have a C5/C6 lesion, but more recently it has been often used by stroke patients who have less disability, yet much more complex movement disorders. The system enables both lateral and palmar grips, using pre-set patterns of stimulation to open and close the hand. Stimulation is triggered by the press of a button mounted on the stimulator case and the splint. The system is designed for holding objects such as a fork or pen for prolonged periods, rather than quick or repetitive movements. The Ness Handmaster is commercially available and FDA cleared.
Although Neuro Muscular Electrical Stimulation (NMES) using surface stimulation has been shown to be effective, there are some disadvantages: attaching electrodes to the skin, and positioning them correctly to achieve the desired movement, discomfort from the sensation of stimulation, and skin irritation of which some have been identified by the users. Implanted and percutaneous FES systems have been used in upper limb applications, most notable are the fully implanted ‘Freehand’ system and the percutaneous system for correction of shoulder subluxation, both developed by Neurocontrol. The Freehand system was shown to be effective in improving hand function in C5/6 tetraplegia, but required extensive invasive surgery with eight epimysial electrodes sutured to the surface of each target muscle and controlled by a receiver/stimulator mounted on the chest wall. It is believed that such systems are unduly difficult to implant due to the required tunnelling of the interconnections between its multiple electrodes and its receiver/stimulator. Additionally, it is believed that such systems are unnecessarily vulnerable to infections (as noted in cardiac pacemakers) that can follow the leads as an infection pathway.
Power signals are transmitted from a coil mounted on the skin over the receiver/stimulator. The subject controls the device through a range of movements of the opposite shoulder, using a skin-surface mounted position detector. Opening and closing of the hand is usually controlled by shoulder retraction and protraction. Strength of grasp is proportional to the degree of movement. A quick elevation or depression of the shoulder activates locking and unlocking of the hand. Two grips are possible: a lateral grasp, where the fingers are first closed and the thumb brought down against the side of the index finger, used for holding small objects such as a pen or fork, and palmar grasp incorporating thumb abduction to hold large objects such as a bottle. The user selects a grasp by pressing a switch, mounted with the shoulder controller. The same switch is also used to turn the system on and off.
The system is intended as a permanent orthosis in subjects who are not expected to experience any natural recovery. Highly significant improvement in hand function has been reported, but the system is limited in its use almost exclusively to C5/6 tetraplegia. It is very expensive and invasive to implant.